Speech communication systems transmit a signal which may contain non-speech signals as well as speech signals. These non-speech signals include noises intentionally made by telephony equipment such as fax machines, and also unwanted noises of indeterminate origin such as squeals and crackles. When converted to sound these non-speech signals may be harmful to a listener such as by causing a listener to experience an acoustic shock. The potential for an acoustic shock is increased when the receiving electro-acoustic transducer is close to the listener's ear such as with a telephone handset. The risk of an acoustic shock is further increased if the receiving electro-acoustic transducer cannot be quickly removed from the ear such as with a headset or headphone. Examples of these speech communications systems are the fixed and mobile telephone system, two-way radios, dictation machines, VoIP systems, hearing aids and intercoms.
There is reason to believe that the phenomenon of acoustic shock adversely affects operatives working in such organisations as call centres, receptionists, and operatives in any area where telecommunication apparatus are used. As well as experiencing pain and discomfort during and for some days after the exposure, some call centre operatives report symptoms of headaches, nausea, tenseness and hypersensitivity to sounds lasting for several days or weeks. In some cases symptoms have been reported to last for several years.
There have been several approaches to minimising the occurrence and intensity of acoustic shocks through means of controlling the acoustic level presented to the listener. These approaches include peak clipping and compression limiting.
Peak clippers control the acoustic level presented to a listener by preventing the voltage applied to the receiving electro-acoustic transducer from exceeding a given magnitude. Peak clipping may be implemented by placing a pair of diodes in reverse polarity to each other across the coil of the receiving electro-acoustic transducer. The frequency content of the acoustic signal resulting from this peak clipping is a function of the receiving electro-acoustic transducer's response. Peak clipping may also be implemented within the circuit that provides the signal to the receiving electro-acoustic transducer. It is possible for this circuitry to provide filtering of the clipped signal prior to applying it to the receiving electro-acoustic transducer. In this case the frequency content of the acoustic signal resulting from this peak clipping is a function of both the filter and the receiving electro-acoustic transducer's response.
Compression limiters control the acoustic level presented to a listener by providing reduced amplification of the signal to be applied to the receiving electro-acoustic transducer when it has a high amplitude. Compression limiting may be implemented by reducing the gain of an amplifier through which the signal passes by the amount the envelope of the input signal exceeds a given threshold. This gain reduction may be performed on a broad band or frequency specific manner and may be followed by filtering. The frequency content of the acoustic signal resulting from compression limiting is a function of the post compression limiting filtering and the receiving electro-acoustic transducer's response. Peak clippers may be used in conjunction with compression limiting.
Both peak clippers and compression limiters can adversely affect the quality and intelligibility of speech. A balance exists between the acoustic protection these devices provide and the effect they have on the quality and intelligibility of speech. As these devices only control the voltage applied to the receiving electro-acoustic transducer any variation in the sensitivity of the receiving electro-acoustic transducer will result in a variation in the resulting sound level presented to the listener. Users of devices are known to interchange transducers. If the user substitutes the transducer for one of higher sensitivity, harmful signal levels may still be presented to the user. The susceptibility to an acoustic shock varies from individual to individual and with the state of the individual. Limiting at a fixed level may offer sufficient protect for a given individual at a given point in time although at other times or for other individuals this level may result in insufficient protection, insufficient speech loudness or speech distortion. Reports of acoustic shock injuries occurring to users of highly controlled limiters indicate that the perceived frequency specific loudness of a signal plays a major part in the causing the injury.